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Summary Campbell Biology H12 Mitosis
Samenvatting van Campbell's Biology H12 Mitosis
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Moleculaire BiologieH12 Mitosis
12.1 Most cell division results in genetically identical daughter cells
The ability of organisms to produce more of their own kind is the one characteristic that best
distinguishes living things from non-living matter. This unique capacity to procreate, like all biological
functions, has cellular basis. The continuity of life is based on the reproduction of cells, or cell
division.
Key Roles of Cell Division
Cell division plays several important roles in life. When a prokaryotic cell divides, it is actually
reproducing because the process gives rise to a new organism (another cell). The same is true of any
unicellular eukaryote. As for multicellular eukaryotes, cell division enables each of these organisms
to develop from a single cell – the fertilized egg. Cell division continues to function in renewal and
repair in fully grown multicellular eukaryotes, replacing cells that die from accidents or normal wear
and tear.
- The reproduction of a cell cannot occur by a mere pinching in half; a cell is not like a soap
bubble that simply enlarges and splits in two. In both prokaryotes and eukaryotes, a crucial
function of most cell divisions is the distribution of identical genetic material – DNA – to two
daughter cells.
Cellular Organization of the Genetic Material
A cell’s DNA, its genetic information, is called its genome. Before the cell can divide to form
genetically identical daughter cells, all of this DNA must be copied, or replicated, and then the two
copies must be separated, so that each daughter cell end sup with a complete genome. The
replication and distribution of so much DNA are manageable because the DNA molecules are
packaged into structures called chromosomes. Each eukaryotic chromosome consists of one very
long, linear DNA molecule associated with many proteins. The DNA molecule carries several
hundred to a few thousand genes, the units of information that specify an organism’s inherited
traits. The associated proteins maintain the structure of the chromosome and helps control the
activity of the genes. Together, the entire complex of DNA and proteins that is the building material
of chromosomes is referred to as chromatin. The chromatin of a chromosome varies in its degree of
condensation during the process of cell division. Each eukaryotic species has a characteristic number
of chromosomes in each cell’s nucleus. For example, the nuclei of human somatic cells (all body cells
except the reproductive cells) each contain 46 chromosomes, made up of two sets of 23, one set
inherited from each parent. Reproductive cells, or gametes – such as sperm and eggs – have half as
many chromosomes as somatic cells; in our example, human gametes have one set of 23
chromosomes.
Distribution of Chromosomes During Eukaryotic Cell Division
When a cell is not dividing, and even as it replicates its DNA in preparation for cell division, each
chromosome is in the form of a long, thin chromatin fiber. After DNA replication, the chromosomes
condense as a part of cell division: Each chromatin fiber becomes densely coiled and folded, making
the chromosomes much shorter and so thick that we can see them with a light microscope.
- Each duplicated chromosome consists of two sister chromatids, which are joined copies of
the original chromosome. The two chromatids, each containing an identical DNA molecule,
are often attached all along their lengths by protein complexes called cohesins; this
attachment is known as sister chromatid cohesion. Each sister chromatid has a centromere,
a region made up of repetitive sequences in the chromosomal DNA where the chromatid is
, attached most closely to its sister chromatid.
This attachment is mediated by proteins that
recognize and bind to the centromeric DNA;
other bound proteins condense the DNA,
giving the duplicated chromosome a narrow
‘’waist’’. The portion of a chromatid to either
side of the centromere is referred to as an arm
of the chromatid. (An unduplicated
chromosome has a single centromere,
distinguished by the proteins that bind there,
and two arms).
Later in the cell division process, the two sister
chromatids of each duplicated chromosome separate
and move into new nuclei, one forming at each end of
the cell. Once the sister chromatids separate, they are
no longer called sister chromatids but are considered
individual chromosomes; this is the step that
essentially doubles the number of chromosomes
during cell division. Thus, each new nucleus receives a
collection of chromosomes identical to that of the
parent cell. Mitosis, the division of the genetic
material in the nucleus is usually followed immediately
by cytokinesis, the division of the cytoplasm.
12.2 The mitotic phase alternates with interphase in the life cycle.
Phases of the Cell Cycle
Mitosis is just one part of the cell cycle, the life of a cell from the time it is formed during division of
a parent cell until its own division into two daughter cells. In fact, the mitotic (M) phase, which
includes both mitosis and cytokinesis, is usually the shortest part of the cell cycle. The M phase
alternates with a much longer stage called the interphase, which often accounts for about 90% of
the cycle. Interphase can be divided into three phases: The G1 phase, the S phase, and the G2
phase.
The G phases were misnamed as ‘’gaps’’ when
they were first observed because the cells
appeared inactive, but we now know that intense
metabolic activity and growth occur throughout
the interphase. During all three phases of the
interphase a cell grow by producing proteins and
cytoplasmic organelles such as mitochondria and
ER. Duplication of the chromosomes, crucial for
eventual division of the cell, occurs entirely
during the S phase. Thus a cell grows (G1),
continues to grow as it copies its chromosomes
(S), grows more as it completes preparations for
cell division (G2), and divides (M). The daughter
cells may then repeat the cycle.
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